The present invention relates to amplifiers and more particularly, to a predistortion type linearized amplifier which can be used in wireless transmitter or the like and can be operated with a low distortion and a high efficiency.
In these years, in an amplifier used in a mobile communication base station or the like, for the purpose of realizing its miniaturization and low cost, it is required to operate a power amplifier to produce a large output with a power efficiency as high as possible.
However, the large-output operational mode is largely influenced by a nonlinear input-output characteristic. Thus a nonlinear distortion takes place outside of a transmission frequency band and produces radio waves disturbing other systems. The amount of such disturbance radio waves is strictly restricted by the Radio Law, which makes it difficult to operate the amplifier with a large output. In order to solve this problem, there are proposes various types of techniques called “distortion compensation”, wherein the power amplifier is made highly linear to reduce the amount of generated nonlinear distortion and to enable its large output operation.
As one of related art distortion compensation techniques, there is proposed a digital predistortion compensating device as disclosed in JP-A-10-145146. In the related art, a complex compensation coefficient is calculated on the basis of amplitude (power) information on a complex plane I+jQ when baseband I, Q signals are regarded as complex signals and the calculated coefficient is complex-multiplied by a baseband complex input signal to realize distortion compensation. However, no consideration is paid to argument of the complex signal. This system is valid for such a type of nonlinear distortion as amplitude modulation to amplitude modulation distortion (AM/AM conversion) known as nonlinear distortion in an ordinary amplifier or as amplitude modulation to phase modulation distortion (AM/PM conversion) which depends only on amplitude.
As mentioned above, the related art is valid only in the case where nonlinear distortion alone depending only on the amplitude with respect to a complex input signal is present. However, when consideration is paid to such a case that, for example, nonlinearity or gain deviation which is independent in each of real and imaginary parts of an orthogonal modulator is present in the orthogonal modulator, the input amplitude of the amplifier will depend not only on the absolute value of the complex input signal but also on the argument thereof. Accordingly, when a combination of the orthogonal modulator and power amplifier is viewed as a whole, compensation is required based on a correspondence relation between complex numbers, which results in difficult compensation with use of the compensation system based only on the amplitude information.
Further, when the power amplifier has a saturation output and the distribution function of the input signal has a highly large amplitude occurrence probability, e.g., follows a normal distribution, it becomes difficult to effectively prevent distortion generated when the large amplitude signal is saturated by the power amplifier.
In the arrangement of the related art, furthermore, a single baseband complex input signal is to be compensated for. Thus the related art arrangement is considered to hard to compensate for distortion generated when transmitting a so-called multicarrier signal obtained by modulating a plurality of complex signals with different carrier frequencies.